1. Field of the Invention
This invention relates to antifouling coating compositions. More particularly, this invention relates to stabilizer compositions that inhibit the increase in viscosity and eventual gelation which often occur during long term storage of antifouling coating compositions containing cuprous oxide and polymers wherein at least a portion of the repeating units are derived from triorganotin salts of ethylenically unsaturated acids.
2. Description of the Prior Art
The attachment and growth of organisms such as barnacles and algae to submerged portions of a ship's hull substantially increases the resistance of the hull to passage through water. The resultant decrease in fuel efficiency and maximum speed of the vessel is sufficient to justify periodic removal of the accumulated fouling. This a costly operation and usually requires that the ship be out of service in a drydock for a substantial period of time. One method commonly employed to inhibit the attachment and growth of fouling organisms to a hull, and thereby extend the period between successive cleanings and repaintings, is to apply a top coat containing an antifouling agent to all submerged portions of the hull. It has long been recognized that triorganotin compounds such as (tri-n-butyltin)fluoride are particularly effective antifouling agents. U.S. Pat. No. 3,167,473 to John R. Leebrick teaches that deficiencies associated with many triorganotin compounds, such as high mammalian toxicity and an excessively high rate of extractability of the triorganotin compound from a paint film by sea water, can be avoided by chemically incorporating the triorganotin compound into a film-forming polymer. In accordance with the teaching of this patent a triorganotin derivative of an ethylenically unsaturated acid such as acrylic or methacrylic acid is polymerized either alone or in the presence of other ethylenically unsaturated compounds, preferably lower alkyl esters of acrylic or methacrylic acid, that will copolymerize with the triorganotin derivative.
U.S. Pat. No. 4,191,579 to G. H. Hails and J. D. Symonds discloses that when an insoluble, seawater-reactive pigment and an insoluble non-reactive pigment are present in an antifouling topcoat applied to a hull, this combination imparts a self-leveling property to the coating as the ship moves through the water. The rate of dissolution is sufficient to release an effective concentration of toxicant while avoiding premature dissolution of the topcoat and the triorganotin-containing polymer employed as both the binder and the antifouling agents. This patent teaches that cuprous oxide and zinc oxide are two of the preferred seawater-reactive pigments. Coatings containing cuprous oxide and a polymer derived at least in part from a triorganotin salt of an ethylenically unsaturated acid are preferred due to the antifouling activity of cuprous oxide, however these coatings cannot be stored for more than six months due to premature gelation of the polymer. The patent discloses that if longer storage periods are required, the organotin-containing polymer and the cuprous oxide must be packaged as separate components. Such a two-package system is undesirable for commercial applications, since it can be difficult to achieve adequate uniform mixing of large amounts of two components. If the final composition is not homogeneous a coating applied using the composition may not contain the same toxicant level over its entire surface. Premature fouling is most likely to occur in those areas of relatively low toxicant concentration.
In accordance with the teaching of U.S. Pat. No. 4,187,211 to A. V. Robinson and J. Chapman, the storage stability of those antifouling compositions disclosed in the aforementioned U.S. Pat. No. 4,191,579 containing cuprous oxide as the seawater-reactive pigment can be substantially increased by including in the composition a relatively inert and water insoluble dehydrating agent such as the anhydrous and hemihydrate forms of calcium sulfate or various water-reactive organic compounds such as the aliphatic isocyanates. Experimental data have demonstrated that in an accelerated aging study conducted at a temperature of 50.degree. C. the viscosity of a coating composition containing anhydrous calcium sulfate, a polymeric triorganotin toxicant and cuprous oxide increased at a faster rate than a control sample in which the calcium sulfate was omitted.